[Bug 793959] qtmux: round to nearest when computing mehd.fragment_duration

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=793959

--- Comment #4 from Alicia Boya García <aboya at igalia.com> ---
Created attachment 374059
  --> https://bugzilla.gnome.org/attachment.cgi?id=374059&action=edit
qtmux: round to nearest when computing mehd and tkhd duration

This fixes a bug where in some files mehd.fragment_duration is one unit
less than the actual duration of the fragmented movie, as explained below:

mehd.fragment_duration is computed by scaling the end timestamp of
the last frame of the movie in (in nanoseconds) by the movie timescale.

In some situations, the end timestamp is innacurate due to lossy conversion to
fixed point required by GstBuffer upstream.

Take for instance a movie with 3 frames at exactly 3 fps.

$ gst-launch-1.0 -v videotestsrc num-buffers=3 \
  ! video/x-raw, framerate="(fraction)3/1" \
  ! x264enc \
  ! fakesink silent=false

dts: 999:59:59.333333334,  pts: 1000:00:00.000000000, duration:
0:00:00.333333333
dts: 999:59:59.666666667,  pts: 1000:00:00.666666666, duration:
0:00:00.333333334
dts: 1000:00:00.000000000, pts: 1000:00:00.333333333, duration:
0:00:00.333333333

The end timestamp is calculated by qtmux in this way:

end timestamp = last frame DTS + last frame DUR - first frame DTS =
  = 1000:00:00.000000000 + 0:00:00.333333333 - 999:59:59.333333334 =
  = 0:00:00.999999999

qtmux needs to round this timestamp to the declared movie timescale, which can
ameliorate this distortion, but it's important that round-neareast is used;
otherwise it would backfire badly.

Take for example a movie with a timescale of 30 units/s.

0.999999999 s * 30 units/s = 29.999999970 units

A round-floor (as it was done before this patch) would set fragment_duration to
29 units, amplifying the original distorsion from 1 nanosecond up to 33
milliseconds less than the correct value. The greatest distortion would occur
in the case where timescale = framerate, where an entire frame duration would
be subtracted.

Also, rounding is added to tkhd duration computation too, which
potentially has the same problem.

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